This invention relates to optical detection systems, particularly those used to measure contaminants present in refillable bottles.
The popularity of refillable containers (typically composed of glass or polymeric materials, e.g., polyethylene terephthalate) has increased, in part due to the environmental and financial costs associated with disposal of containers. After use, refillable containers may be returned to a bottling plant for cleaning and inspection before being refilled. This inspection screens the containers for physical damage (e.g., cracking) and contaminants (e.g., hydrocarbons and detergents) that might degrade the flavor, safety, or other qualities of the final product. The risk of contamination is particularly high in containers made from plastic, rather than glass, because contaminants tend to absorb into the plastic walls of the container, and may leach into the product despite cleaning procedures.
It is usually desirable, therefore, to test the container for trace amounts of contaminants prior to refilling. Such tests are preferably rapid and non-invasive in order to be efficiently incorporated into high-volume refilling processes involving assembly lines.
An existing contaminant-testing method involves the use of a gas nozzle configured to deliver pressurized air or another suitable gas to the containers, thereby displacing a portion of the gas-phase contaminants contained within. The displaced contaminants are then imported as a sample to one or more detectors, such as an optical excitation/detection mechanism, positioned at a point away from the point of samplings. Such systems are used to optically monitor the presence of the contaminants, and typically include a light source (e.g., a laser or flashlamp) which is used to optically excite the gas-phase contaminants in the sample. As the excited molecular components of the contaminants return to their ground states, they emit a characteristic fluorescence with an intensity linearly proportional to their concentration. The induced fluorescence is then imaged onto a photodetector using a lens or an equivalent optical system.
When such a system is used to detect trace amounts of contaminants, it is usually desirable to maximize the signal-to-noise ratio of the detected signal. Unfortunately, dust or contaminants continuously drawn into the housing containing the excitation/detection system may collect on the various optical components (particularly on glass-based components, such as lenses and mirrors), causing attenuation of the detected signal. Filtration of the sample to remove debris is typically impractical as this process may also remove the contaminant to be detected. Additionally, the presence of debris tends to increase the magnitude of the diffuse scattered excitation light, which in turn increases the amount of detected noise. When the build-up of these foreign materials is such that an adequate signal-to-noise ratio in the measured signal cannot be achieved, the assembly line must be stopped in order for the optical components to be cleaned; this process is undesirably frequent when such fluorescence detectors are used in dirty environments.